This technology involves rear projection screens, and in particular rear projection screens having an array of transparent refractive elements in an opaque matrix.
A rear projection screen is a sheetlike optical device having a relatively thin viewing layer which is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by most projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
A well-known type of rear projection screen is a thin, light diffusing layer such as a frosted or translucent glass surface, which may be produced by etching, sandblasting, or otherwise roughening a smooth glass surface. The translucent surface limits the visibility of objects behind the screen. The screen must, however, be sufficiently light transmissive to allow the projected image, which is focused precisely on the translucent surface, to be viewed from the front side of the screen. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent suffer, however, from a tendency to strongly reflect ambient light incident on the front side, thereby causing fading, or washout, of the projected image. This problem is particularly severe if the background or ambient light is bright.
An approach to reducing the effects of ambient light while still maintaining an acceptable level of projected image light is disclosed in U.S. Pat. No. 5,563,738, wherein an array of closely packed glass beads is attached to a transparent substrate by an opaque polymeric binder. The glass beads act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the beads. The foci are in the areas where the beads contact the front support layer.
Because the transparent beads contact the transparent front support layer, they exclude most of the opaque binder material from the space between the beads and their contact areas on the substrate. This forms an optical aperture between each bead and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the bead interstices. As a result, ambient light incident in these areas is absorbed. Thus the front side of the screen appears black, except for the light transmitted through the beads. Further improvements to the screen are disclosed wherein an additional opaque layer may be applied to the interstices between the beads on the rear side of the screen, and an additional transparent layer may be applied to the beads on the rear side of the screen, thereby altering their optical performance.
Glass bead rear projection screens suffer, however, from certain limitations, especially when they are used to project high quality images that are to be viewed from short distances. More particularly, the appearance of such screens is highly sensitive to the quality and placement of the glass beads used. Beads that are of incorrect size, are not spherical, or are broken, nicked, scratched, or otherwise defective can create a variety of visible defects, variously called graininess, scintillation, sparkles, speckle, punch through, or simply spots. These defects are particularly troubling when the screen is used, for example, as a computer monitor, where the need for seeing a high level of detail is likely to lead the user to scrutinize the screen closely, from a short distance, for long periods of time.
According to the present invention, a rear projection screen includes a plurality of refracting beads bound in optical contact with a substrate. The beads are surrounded by an opaque matrix. A diffuser is positioned such that light traveling through the screen will pass through the diffuser after passing through the beads but before exiting the screen.